This invention relates to methods for producing iron oxide particles in a solid non-magnetic matrix by rapid solidification of an iron oxide precursor mixed with a non-magnetic matrix material.
Oxide particles for magnetic recording purposes are currently synthesized using a very complex and lengthy procedure. This procedure requires the precipitation of synthetic .alpha.-(FeO)OH(goethite) from aqueous solutions, the dehydration of .alpha.-Fe.sub.2 O.sub.3 to Fe.sub.3 O.sub.4, and finally the careful oxidation of Fe.sub.2 O.sub.4, to form .gamma.-Fe.sub.2 O.sub.3. (Akashi, Ferrites: Proceedings of the International Conference, Sept.-Oct. 1980 Japan p. 548-552).
Another wet chemistry technique for preparation of iron oxide (spinel ferrite) powders consists of precipitation of the spinel ferrite from aqueous solutions containing ferrous ions and other divalent metallic ions. The solution pH is controlled formed by oxidation of the aqueous solution in air above 50.degree. C. (Takada, Ferrites: Proceedings of the International Conference. Sept.-Oct., 1980, Japan, p. 3-6).
Another wet chemical method for synthesis of iron oxide powders involves the reduction of ferrous ions with sodium borohydride, a very expensive reducing agent, in the presence of a magnetic field (Akashi, Ferrites: Proceedings of the International Conference, Sept.-Oct., 1980, Japan, p. 548-552).
It is also possible to prepare cobalt-ferrite iron oxide powders using wet chemistry methods. According to one such method, acicular .gamma.-Fe.sub.2 O.sub.3 particles are suspended in an alkaline solution containing Co.sup.2+ and then treated at 90.degree. C. for 10 hours (Hayama, Ferrites: Proceedings of the International Conference, Sept.-Oct., 1980, Japan, p. 521-525).
Alternative preparation methods for iron oxide powders include condensation of vaporized metal in a low pressure inert gas atmosphere in the presence of a magnetic field. Powders produced in this manner exhibit low noise levels and excellent stability against oxidation; however, this method is extremely expensive (Akashi, Ferrites: Proceedings of the International Conference, Sept.-Oct., 1980, Japan, p. 548-552).
Cobalt modified iron oxides may be prepared using pyrolytic decomposition (chemical vapor deposition) of cobalt-acetylacetonate vapor on the surface of iron oxide fluidized acicular particles. Powders prepared using this process are suitable for magnetic recording applications. They exhibit high coercivities (550-600 Oe) at relatively low Co.sup.2+ (2-3) and Fe.sup.2+ (8-9) wt % (Monteil et al. Ferrites: Proceedings of the International Conference, Sept.-Oct., 1980, Japan, p. 532-536).
Complex oxides for ferrites, consisting of iron oxide and zinc oxide, are prepared by spraying aqueous chloride solutions of the respective oxide metallic constituents onto a fluidized roasting furnace (Hirai et al. WO88-00925).
The iron oxide and doped iron oxide particles produced according to these methods are incorporated into magnetic discs, tapes and other devices for magnetic recording. Such magnetic recording media usually consist of a non-magnetic support for a magnetic recording layer of ferromagnetic powder (i.e. iron oxide or doped iron oxide) dispersed in an organic binder material (Saito et al., U.S. Pat. No. 4,820,581, JP 62125533, JP 57078631, JP 62241134, JP 62162228, and Funahashi et al. U.S. Pat. No. 4,820,565) where the non-magnetic support or substrate may be a polyester film or tape.
Efforts have also been directed towards improvement of durability of magnetic recording media. One approach involves the combination of cobalt doped .gamma.-Fe.sub.2 O.sub.3 and Cr.sub.2 O.sub.3 with non-magnetic .alpha.-iron oxide in an organic binder by ball milling subsequent coating of a PET film (JP 55129935).
Other approaches coat the magnetic iron oxide particles with a layer of silica (SiO.sub.2) by sputtering (JP 58006528) or by immersing the metal oxide particle in a ph-controlled suspension with an amorphous, powdered silicate which is subsequently made crystalline by adjustment of the suspension ph. An alumina (Al.sub.2 O.sub.3) protective coating may also be e-beam deposited on the surface of magnetic iron oxide particles (JP 58006528). A non-magnetic zinc oxide (ZnO) protective coating has also been electro-deposited on a magnetic iron oxide layer which had previously been electro-deposited on an aluminum or aluminum alloy substrate (JP 621255126).
Glass-forming additives such as B.sub.2 O.sub.3, Bi.sub.2 O.sub.3, P.sub.2 O.sub.5, MoO.sub.3 and V.sub.2 O.sub.5 have been used in the solid state reaction of .alpha.-FeOOH particles and colloidal BaCo.sub.3 or SrCO.sub.3.
Addition of approximately 0.5 wt % B.sub.2 O.sub.3 best accelerated ferrite formation without adhesion of particles, resulting in thin-plate hexagonal ferrites with good magnetic recording properties (Sugimoto, Fourth International Conference on Ferrites, Part 2, Oct.-Nov. 1984, San Francisco, Calif., U.S.A. p. 273-279).
Amorphous cobalt ferrite (CoFe.sub.2 O.sub.4) films have also been prepared using a two-source vacuum evaporation technique with CoFe.sub.2 alloy and P.sub.2 O.sub.5 as the source materials. These amorphous cobalt ferrite films display high perpendicular anisotropy (Hiratsuka et al., Electronics and Communications in Japan, Part 2, 71, 95-102 (1988) and IEEE Transactions on Magnetics, MAG-23, 3326-3328 (1987)).
Japanese patent, JP 61080618, also describes sputtering of Co-.gamma.-Fe.sub.2 O.sub.3 on a non-magnetic disc substrate. Japanese patent, JP 62095735, describes a process for conversion of sputtered .alpha.-Fe.sub.2 O.sub.3 to Fe.sub.3 O.sub.4 on a non-magnetic substrate such as a drum or disc, using laser beam irradiation.
Methods exist for precipitation or nucleation of magnetic crystallites in glassy matrices.
U.S. Pat. No. 4,083,727 to Andrus et al. discloses a method for production of glass-ceramic articles having integral magnetic magnetite (Fe.sub.3 O.sub.4) crystals. According to this method, a Li.sub.2 O--FeO--Al.sub.2 O.sub.3 --SiO.sub.2 glass article nucleated with TiO.sub.2 is heat-treated to induce crystalline nucleation within the article body, resulting in a glass-ceramic article which is subsequently exposed to a reducting atmosphere to convert hematite crystals in its surface layer to magnetite, yielding films with high coercivities and saturation magnetizations which compare favorably to those of magnetite and other ceramic ferrite materials. The magnetic recording medium disclosed in JP 62042315 consists of a layered structure, one layer of which is a magnetic recording medium made from .alpha.-FeO.sub.2 O.sub.3, Al.sub.2 O.sub.3, SiO.sub.2, B.sub.2 O.sub.3 or Co.sub.3 O.sub.4.
The nucleation of inhomogeneous precipitates having ferrimagnetic cores within antiferromagnetic skins has been observed in B.sub.2 O.sub.3 --BaO--Fe.sub.2 O.sub.3 glass matrices prepared by slow quenching between stainless steel slabs (Fahmy et al., Physics And Chemistry Of Glasses, 13, 21-26 (1972) and MacCrone, in Amorphous Magnetism, H. O. Hooper and A. M. deGraaf, eds., Plenum Press, New York-London 1973 p. 77).
Magnetic phases have been precipitated in Li.sub.2 B.sub.2 O.sub.4 --LiFe.sub.5 O.sub.8 glass systems prepared by splat quenching, roller quenching and gun quenching techniques (Chaumont et al., Mat. Res. Bull. 15, 771-776 (1980); Chaumont et al., Rapidly Quenched Metals III, Third International Conference, University of Sussex, Brighton, July 1978 p. 401; Chaumont et al., Rev. Int. Htes. Temp. et Refract. 15, 23-32 (1978)).
Partially recrystallized glasses in the Li.sub.2 O--Fe.sub.2 O.sub.3 SiO.sub.2 system, prepared by slow quenching between steel plates exhibit some ferrimagnetic properties at high Fe.sub.2 O.sub.3 contents (Weaver et al., American Ceramic Society Bulletin 52, 467-472 (1973)).
Ferrimagnetic amorphous cobalt ferrites have been prepared by rapid quenching to liquid nitrogen temperatures of cobalt ferrite combined with P.sub.2 O.sub.5 glass network former (Sugimoto et al., Jpn. J. Apl. Phys. 21, 197-198 (1982)).
Splat quenching techniques have been applied to the BaO--Fe.sub.2 O.sub.3 and SrO--Fe.sub.2 O.sub.3 systems where BaFe.sub.12 O.sub.19 crystals have been observed along with weak ferromagnetism in the glass matrix (Monteil et al., Mat. Res. Bull. 12, 235-240 (1977); Monteil et al., Journal of Solid State Chemistry, 25, 1-8 (1978); Chaumont et al., Mat. Res. Bull. 15, 771-776 (1980); Chaumont et al., Rapidly Quenched Metals III, Third International Conference, University of Sussex, Brighton, July 1978 p. 401; Chaumont et al., Rev. Int. Htes. Temp. et Refract. 15, 23-32 (1978)).